Thanks to their outstanding mechanical properties, yarns based on homopolymers or copolymers obtained from C2+n monomers, wherein n varies from 0 to 2, such as ultra high molecular weight polyethylene and high molecular weight polypropylene homopolymer, have been used in special applications, which require that the material be able to withstand high deformation stresses, at the time of use. Such situations discussed herein, where high deformation stresses are present, can be the such of explosions or ballistic impacts.
One of the pioneering processes for the manufacture of fibers from ultra high molecular weight polyethylene (UHMW PE) was patented by the company Stamicarbon in 1977, which is described in U.S. Pat. No. 4,137,394. In this patent, a vessel was used for the dissolution of the UHMW PE in a first solvent, so that an extruded monofilament could be obtained from a single or twin screw extruder, and said monofilament treated in a bath with a second solvent, in a continuous manner. This process was later improved, and three additional patents were obtained by the same company in 1980: U.S. Pat. No. 4,422,993, GB 2,042,414 and GB 2,051,667. According to these patents, the polymer dissolution process was carried out in batches.
At the beginning of the 80s, Allied Corporation was granted patents for the continuous manufacture of UHMW PE yarns process for the, as described in patents U.S. Pat. No. 4,413,110 and U.S. Pat. No. 4,663,101. According to this Allied Corporation's process, the gel formation occurs in a vessel that feeds the extruder, which is in line with a fiber extraction and drying system. Allied Corporation was also granted other patents related to the manufacture of high performance ballistic fabrics using UHMW PE yarns (U.S. Pat. No. 4,403,012, U.S. Pat. No. 4,876,774, U.S. Pat. No. 4,681,792 and U.S. Pat. No. 4,916,000).
Since then, a series of improvements has been made to the process for producing polymeric yarns, aiming to obtain a higher tenacity, higher elasticity modulus and lower elongation material.
U.S. Pat. No. 4,663,101 patent assigned to Allied Corporation discloses a process in which the polymer is mixed with a first paraffinic oil solvent, in a mixing vessel, so as to form a suspension. This suspension is then transferred to a pre-heater, where a 5 to 50%, by weight, dissolution of the polymer occurs. In sequence, the mixture is fed to the spinneret bearing extruder, said mixture passing through an open air space on to a cooling bath. In line with the cooling system, the filament bundle runs through an extraction system with a second solvent running in counter-flow with the filaments and later through a drying system. The first solvent is extracted from the yarn by a second solvent which is later evaporated from the yarn. The dry yarn is stretched in at least two steps, the last one being operated at temperatures between 135° C. and 150° C.
U.S. Pat. No. 4,668,717 patent assigned to Stamicarbon describes a UHMW PE fiber production process, in which a suspension of UHMW PE particles is prepared in an appropriated solvent, at room temperature, and this suspension is transformed into a homogeneous solution inside a single or twin screw extruder, where the mixture is subjected to high mechanical shear, thereby causing the dissolution of the polymer in the solvent. It is necessary that the extruder be operated at relatively high screw rotations, so that the residence times inside it be small and the degradation of the polymer does not accompany its dissolution in the solvent.
U.S. Pat. No. 6,689,462 B2 patent, assigned to DSM, describes the influence of a polymer's narrow molecular weight distribution on its processability, in the case of gel spinning process. To promote better spinning in a gel spinning process, two types of polyethylene (PE) were used, namely: a high molecular weight polyethylene (HMW PE) and an ultra high molecular weight polyethylene (UHMW PE), in the same mixture, taking into account each polymer's intrinsic viscosity.
U.S. Pat. No. 5,230,854 patent, assigned to Allied Corporation, describes a continuous process for the removal of the spinning solvent, which rests entrapped within the yarns. This process comprises the extraction of the spinning solvent by means of a second solvent, preferably a volatile second solvent, in an extraction vessel, said polymer being insoluble in the second solvent. The yarn must have a residual oil content, which is the spinning solvent, also called gelling solvent, of less than 1.5%. Thus, the solution containing the first solvent, which is the spinning solvent, and the second solvent, which is the extraction solvent, is led to a solvent separation equipment, in which a third solvent is added to the solution, said third solvent being capable of extracting the second solvent, forming a two phase mixture. The separated spinning solvent is returned to the beginning of the spinning process, and the solution containing the second and third solvents is led to another equipment, where the recovery of the extraction solvent will take place, possibly by temperature induced phase separation. In case the extraction solvent, called the second solvent, is not volatile, the yarn containing the second solvent will have to be washed with the third solvent, which will be evaporated in a later step.
U.S. Pat. No. 7,147,807 patent, assigned to Honeywell, describes a process utilizing an ultra high molecular weight homopolymer or copolymer dissolved in decalin at a temperature below 250° C., under not inerting conditions, the resulting extruded yarn being cooled in a cooling solution and dried by one or more gas streams, without the use of a second solvent. The solvent, in this case decalin, is used at a temperature close to its boiling point, 250° C., which in turn makes for the expansion of the liquid exiting the extruder, forming vapors which are absorbed by a gas stream which contacts the filaments at a number of stages, until the complete reduction of the gelling solvent concentration to an acceptable level. The filament is then stretched in two stages. A number of processes for the recovery of low molecular weight gelling solvent are claimed in this patent. Besides being complex, the process worries about the explosivity limit of decalin in air at 100° C., which is 0.7%, by vol. In these processes, the vapors are treated in complex recovery systems, on top of presenting the inconvenience of using relatively toxic solvents.